**Correct Option: B. Molecules in solids are closely packed and vibrate in place, while those in gases are widely spaced and move freely.** ### Detailed Explanation: To understand the primary difference between solids and gases at the molecular level, we need to look at how the molecules in each state of matter behave and are arranged. 1. **Molecular Arrangement**: - **Solids**: In solids, the molecules are closely packed together in a fixed arrangement. This close packing is due to strong intermolecular forces (like ionic, covalent, or metallic bonds) that hold the molecules in place. The molecules do not have the freedom to move around; instead, they can only vibrate in their fixed positions. This is why solids have a definite shape and volume. - **Gases**: In contrast, the molecules in gases are much farther apart compared to those in solids. The intermolecular forces in gases are very weak, allowing the molecules to move freely and independently of one another. This freedom of movement is why gases do not have a definite shape or volume; they expand to fill the container they are in. 2. **Molecular Motion**: - In solids, while the molecules are in constant motion, this motion is limited to vibrations around their fixed positions. The kinetic energy of the molecules in solids is lower compared to gases. - In gases, the molecules are in constant, rapid motion, moving in straight lines until they collide with other molecules or the walls of their container. This high kinetic energy allows gases to fill any available space. 3. **Intermolecular Forces**: - The strong intermolecular forces in solids keep the molecules tightly packed, which is why solids maintain their shape. - In gases, the weak intermolecular forces mean that the molecules can move freely, leading to a lack of defined shape and volume. ### Why Other Options Are Incorrect: - **Option A**: "Molecules in solids are in constant motion, while those in gases are at rest." - This statement is incorrect because it implies that gas molecules are at rest, which is not true. Gas molecules are in constant motion, moving freely and rapidly. - **Option C**: "Molecules in solids have no intermolecular forces, while those in gases are held together by strong forces." - This statement is fundamentally incorrect. Solids have strong intermolecular forces that keep their molecules closely packed, while gases have weak intermolecular forces that allow for free movement. - **Option D**: "Molecules in solids have more kinetic energy than those in gases." - This statement is also incorrect. In fact, gas molecules have more kinetic energy than solid molecules due to their rapid motion. The kinetic energy of a substance is directly related to the temperature and the freedom of movement of its molecules. ### Summary of Key Points: - **Molecular Arrangement**: Solids have closely packed molecules that vibrate in place; gases have widely spaced molecules that move freely. - **Molecular Motion**: Solids vibrate around fixed positions; gases are in constant, rapid motion. - **Intermolecular Forces**: Solids have strong intermolecular forces; gases have weak intermolecular forces. - **Kinetic Energy**: Gas molecules have higher kinetic energy than solid molecules due to their freedom of movement. This understanding of the differences between solids and gases at the molecular level is crucial for grasping the fundamental concepts of states of matter in physics.